Measuring the Radius of Curvatire using a Spherometer
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... turn of the dial also corresponds to 0.5 mm and each small graduation on this dial
represents 0.005 mm. The small graduations on the old spherometer are 0.001 ...

Re: ATM Micrometer Head
... They list a Brown and Sharp for $65.00 with a 0.0001 graduation on page 1587. ... a source
for a good-quality micrometer head (for making a >spherometer), one that ...
astro.umsystem.edu/atm/ARCHIVES/FEB96/msg00259.html

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Note added at 2004-02-19 11:45:19 (GMT)
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Spherometers are used to measure the radius of curvature of the surface of a lens.
When the three outer legs are placed on a flat surface and the central leg screwed down to touch the surface, the zero reading is obtained.

The vertical scale reads in millimeters, and the scale around the edge of the circular disk may be divided into as many as 1000 GRADUATIONS. [...]


http://physics.kenyon.edu/EarlyApparatus/Optics/Spherometer/...